Introducing multiple treatment plan-based comparison to investigate the performance of gantry angle optimisation (GAO) in IMRT for head and neck cancer

Figures & data

Table I. Dose planning specific parameters: prescribed dose to the PTV (Gy), PTV volume (cm³), parotid gland volume (cm³), overlap between the parotid gland volume and the PTV (cm³, %).

Case	Diagnose	Prescribed dose (Gy)	PTV (cm^3)	Parotid (cm^3)	Overlap (cm^3)	Overlap/Parotid (%)
1	Oropharynx	68	268	25	0.20	0.80
2	Oropharynx	66	117	25	0.40	1.6
3	Epipharynx	68	240	9.3	0.80	8.6
4	Oropharynx	66	95	22	6.4	29

Table II. Machine specific parameters: minimum segment size (cm³) in OMP, smoothing parameters (x,y) in Eclipse, number of monitor units, MU with and without GAO in Eclipse and OMP, and the decrease in number of monitor units, MU, (%) by using GAO in both Eclipse and OMP.

Case	Min seg. size (cm^3)	Smoothing (x,y)	MU	MU	Decrease (%)	MU	MU	Decrease (%)
	OMP	Eclipse	OMP	OMP GAO	OMP	Eclipse	Eclipse GAO	Eclipse
1	5	60	629	620	1.43	1080	916	15.3
2	5	60	493	421	14.6	728	683	6.18
3	5	60	833	809	2.88	902	694	23.1
4	5	60	604	506	16.2	1080	775	28.4

Figure 1. The average absorbed dose (Gy) to the parotid gland, <D>_{parotid gland}, in the trade-off as a function of the under-dosage to the PTV, V_{PTV,D < 95%}, with equidistant beam geometry and GAO in Eclipse and OMP.

Figure 2. The average absorbed dose (Gy) to the parotid gland, <D>_{parotid gland}, in the trade-off as a function of the dose homogeneity (D₅-D₉₅) in the PTV.

Table III. The number of treatment plans generated with and without GAO in Eclipse and OMP.

Number of plans Case	Eclipse	Eclipse GAO	OMP	OMP GAO
1	10	10	11	12
2	7	6	10	6
3	6	5	6	8
4	6	9	10	9
Total	29	30	37	35

Figure 3. The beam geometries obtained using GAO in the OMP system (black dots), in the Eclipse system (black dashed lines) and the equidistant beam geometry (grey solid lines) for the four cases calculated as average beam geometries for all treatment plans generated. The colours for the ipsilateral parotid/PTV in the trade-off are for Case 1 (upper left) blue/ red, Case 2 (upper right) blue/red, Case 3 (lower left) pink/red and Case 4 (lower right) green/red.